Protein-linked, high mannose-type oligosaccharides devoid of glucose residues are glucosylated directly from UDP-Glc in the lumen of the endoplasmic reticulum (ER) to form monoglucosylated derivatives. In vivo the glucose units are immediately removed by glucosidase Il. The glucosylating enzyme (UDP-Glc:glycoprotein glucosyltransferase) has the unique property of glucosylating denatured but not native glycoproteins in cell-free assays. We have purified to homogeneity the enzyme from rat liver and the yeast Schizosaccharomyces pombe and cloned, sequenced and disrupted the gene coding for it in the last organism. It has been proposed that the enzyme participates, together with an ER chaperone that binds monoglucosylated oligosaccharides (calnexin) in the quality control mechanism of glycoprotein folding by which only species having the correct tertiary structures are transported to the Golgi apparatus. As the participation of the glucosyltransferase is solely based in the above mentioned property but not in in vivo performed experiments, we propose to confirm or disprove the proposed involvement of the enzyme in the quality control mechanism by performing the following experiments: a) To characterize, sequence and disrupt the gene encoding the glucosyltransferase in Trypanosoma cruzi and to study glycoprotein folding, secretion and association with calnexin in the genetically modified protozoon. Trypanosomatids are the only organisms known so far that transfer unglucosylated oligosaccharides in protein N-glycosylation and therefore, the only mechanism by which they glucosylate proteins is that involving the glucosyltransferase. b) To study calnexin-association, folding and secretion of glycoproteins in S. porn be glucosyltransferase minus cells. As these cells but not calnexin minus ones proved to be viable, to study if disruption of another gene in addition to that encoding the glucosyltransferase renders S. pombe cells non-viable. c) As we have determined that the glucosyltransferase binds hydrophobic amino acids, to study the possibility that in vivo the enzyme behaves as a chaperone and to further characterize the hydrophobic domains recognized by the glucosyltransferase in denatured glycoproteins.